This invention relates to electrical storage cells, and, more particularly, to sodium sulfur cells for use in a weightless environment.
Rechargeable cells or batteries are electrochemical devices for storing and retaining an electrical charge and later delivering that charge for useful power. Familar examples of the rechargeable cell are the lead-acid cell used in automobiles and the nickel-cadmium cell used in various portable electronic devices such as cameras and radios. Another type of cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, known as the nickel-hydrogen cell, which is used in spacecraft.
Yet another type of cell is the sodium sulfur cell, which has been under development for about 20 years for use in a variety of terrestrial applications such as nonpolluting electric-powered vehicles and utility load levelling. The sodium sulfur cell has the particular advantage that its storage capacity per unit weight of cell is nearly three times, and in some designs as much as five times, the storage capacity of the nickel-hydrogen cell. The sodium sulfur cell therefore is an attractive candidate for use in spacecraft.
The most common type of construction for a sodium sulfur cell includes a cylindrical metal outer housing which serves as a positive terminal, and a cylindrical shell of sodium beta/beta-double prime alumina ceramic within the metal housing. Sodium is placed into a first or inner chamber formed within the alumina shell, and sulfur is placed into a second chamber formed between the alumina shell and the outer housing. The cell is heated to a temperature of about 350.degree. C., at which temperature both the sodium and the sulfur are molten. The liquid sodium acts as the anode of the cell, the liquid sulfur acts as the cathode, and the solid beta/beta-double prime alumina ceramic acts as the electrolyte. Electrical energy is released when sodium ions diffuse through the ceramic into the sulfur, thereby forming sodium polysulfides. Electrical energy can be stored when the process is reversed, with an applied voltage causing the sodium polysulfides to decompose to yield sodium and sulfur. The sodium ions diffuse through the ceramic electrolyte back into the first chamber.
The sodium sulfur cell is a candidate for use in energy storage for spacecraft such as communications satellites. A satellite orbiting the earth is periodically exposed to direct sunlight and then plunged into shadow. In many satellites, electrical energy to power the systems on board the satellite is created by solar cells that function when the satellite is in sunlight, and some of the energy so generated is stored in electrical storage cells. The stored energy is then available for use when the satellite is in the earth's shadow or for peak power demands, by discharging the cells.
Nickel-cadmium and nickel-hydrogen electrical storage cells are currently used in many satellite applications. Such cells have the capacity to store about 17-18 watt hours per pound of cell weight, or less. A sodium sulfur cell has the capacity to store over 50 watt hours per pound of cell weight using existing cell designs. In one example, about 670 pounds of nickel-hydrogen cells are required in a communications satellite to meet its storage needs. If the nickel-hydrogen cells were replaced by sodium sulfur cells, the weight of storage cells would be reduced to about 250 pounds. The weight of the cells is included in the cost of launching the satellite, which presently is on the order of $20,000 per pound, and a potential reduction of over 400 pounds is highly significant.
Although sodium sulfur cells offer potential benefits in spacecraft applications, their operation has been established only on earth. A key difference between operation in a terrestrial environment and in a spacecraft is the absence of gravity in space. It has been determined that the absence of gravity may have significant adverse effects on the functioning of the cell, particularly under fast discharge conditions, that are not experienced in earthbound applications. For example, the rate of discharge is important in satellites orbiting in low earth orbit, as the relatively short periods of time in shadow and applications power needs require the batteries to deliver their stored energy rapidly. There is a need to develop an approach to avoiding the expected adverse effects of weightlessness on the ability of the cells to discharge rapdily. The present invention fulfills this need, and further provides related advantages.